AN-5841 Datasheet, PDF (1/8 Pages) Fairchild Semiconductor – Applying SG5841 to Control

www.fairchildsemi.com
AN-5841
Applying SG5841 to Control a Flyback Power Supply
Summary
This application note describes a detailed design strategy for
a high-efficiency, compact flyback converter. Design
considerations, mathematical equations, and guidelines for a
printed circuit board layout are provided.
Features
ƒ Green-Mode PWM Controller
ƒ Low Startup Current: 14µA
ƒ Low Operating Current: 4mA
ƒ Programmable PWM Frequency with Hopping
ƒ Peak-Current-Mode Control
ƒ Cycle-by-Cycle Current Limiting
ƒ Synchronized Slope Compensation
ƒ Leading-Edge Blanking (LEB)
ƒ Constant Output Power Limit
ƒ Totem-Pole Output with Soft Driving
ƒ V_DD Over-Voltage Clamping
ƒ Programmable Over-Temperature Protection (OTP)
ƒ Internal Open-Loop Protection
ƒ V_DD Under-Voltage Lockout (UVLO)
ƒ GATE Output Maximum Voltage Clamp: 18V
Description
The SG5841 is a highly integrated PWM controller IC. It
provides features to satisfy the need for low standby power
consumption. With low startup current and low operating
current, high-efficiency power conversion is achieved.
Typical startup current is only 14µA and operating current is
around 4mA. In nominal loading conditions, the SG5841
operates at fixed PWM frequency. As the load decreases, its
proprietary green-mode circuit gradually reduces the PWM
frequency. This green-mode function dramatically cuts the
power loss in no-load and light-load conditions, enabling the
power supply to meet power conservation requirements.
Additionally, the controller incorporates many protection
functions. Once the power supply is overloaded, the
controller forces the power supply into “hiccup” mode to
limit output power. The built-in line-voltage compensation
circuit maintains constant maximum output power for a wide
input line voltage range. An external negative-temperature-
coefficient (NTC) thermistor can be connected to the RT pin
for over-temperature protection.
Figure 1. Pin Configuration
© 2006 Fairchild Semiconductor Corporation
Rev. 1.1.1 • 4/24/08
www.fairchildsemi.com